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Content for  TR 23.700-89  Word version:  18.0.0

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6.9 Solution #9: PCF-updated RFSP Index to MME via HSS  6.9.1 Description  6.9.2 Procedures  6.9.3 Impacts on services, entities and interfaces 
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6.9  Solution #9: PCF-updated RFSP Index to MME via HSS  p. 25

6.9.1  Description  p. 25

This solution address KI#1 and applies to UE interworking between 5G and 4G, including:
  • 5G to 4G mobility with N26 or without N26.
  • 4G to 5G mobility with N26 or without N26.
To direct the UE from 5G to 4G in some scenarios, the PCF may adjust the RFSP index for the UE and notify it to the AMF to update the RFSP index in use for the UE, for example:
  • Based on some statistics and prediction result from NWDAF F (e.g. Observed Service Experience analytics, UE communication analytics, Network Performance analytics for 5G, etc.), the PCF may request to move some of UEs to 4G to reduce the 5G network congestion.
  • Bases on application in use, e.g. if the application in use for the UE is suitable to be performed in 4G, then the PCF may request to move the UE to 4G for better service experience.
The PCF notifies the updated RFSP index to the UDM/HSS.
The UDM/HSS will send the updated RFSP index to the MME serving the UE in the Location Update Response message during the TAU or attach procedure, which is initiated by the UE after accessing to 4G.
Based on the updated RFSP index received from HSS, the MME sets RFSP index in use for the UE, which is consistent with AM policy in 5G and allows the UE to stay in 4G.
After the UE is directed from 5G to 4G, the PCF may adjust the RFSP index again to request 4G network to direct the UE back to 5G according to dynamic network situations, for example:
  • Based on some statistics and prediction result from NWDAF F (e.g. Observed Service Experience analytics, UE communication analytics, Network Performance analytics for 5G, etc.), the PCF may request to move some of UEs to 5G when the 5G network congestion has eased
  • Bases on application in use, e.g. if the application in use for the UE is suitable to be performed in 5G, then the PCF may request to move the UE to 5G for better service experience.
In a similar way, the PCF notifies the updated RFSP index to the UDM/HSS, and the UDM/HSS will notify the updated RFSP index to the MME by reusing the Insert Subscribe Data procedure. Then based on the updated RFSP index received from HSS, the MME sets RFSP index in use for the UE. The RFSP index in use will be sent to eNB, which will guide the UE to move from 4G to 5G.
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6.9.2  Procedures  p. 26

6.9.2.1  Procedure for triggering UE to move from 5G to 4G  p. 26

Copy of original 3GPP image for 3GPP TS 23.700-89, Fig. 6.9.2.1-1: PCF-updated RFSP Index update from 5GC to MME in N26-base architecture
Figure 6.9.2.1-1: PCF-updated RFSP Index update from 5GC to MME in N26-base architecture
(⇒ copy of original 3GPP image)
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Step 1.
The UE registers to 5GC via NG-RAN. AMF establishes AM Policy association with PCF as in clause 4.16.1.2 of TS 23.502.
Step 2.
The PCF decides to adjust the RFSP index to direct the UE from 5G to 4G. The PCF decision could be based on various inputs. For example:
  • Based on various analytics result from NWDAF (e.g. Observed Service Experience analytics, UE communication analytics, Network Performance analytics for 5G, etc.).
  • Bases on application in use, e.g. the application in use is suitable to be performed in 4G.
Step 3.
The PCF updates RFSP index for the UE to RFSP index value 1, and provides the RFSP index vaule1 to the AMF by initiating AM Policy Modification procedure as in clause 4.16.2.2 of TS 23.502.
Step 4.
The AMF provides the RFSP index vaule1 as the RFSP index in use to the gNB by initiating a UE CONEXT MODIFICATION procedure as in clause 8.3.4 of TS 38.413.
Step 5.
Then the PCF notifies the UDM/HSS the RFSP index vaule1. If the UDM and HSS are deployed separately then the UDM further notifies the RFSP index vaule1 to the HSS. In a combined UDM+HSS deployment this step (5b in Figure 6.9.2.1-1) is not needed or handled in implementation specific way.
Step 6-7.
When the UE accesses to 4G, the UE performs TAU or Attach procedure, during which the HSS provides RFSP index vaule1 to the MME serving the UE.
Step 8.
The MME updates RFSP index in use for the UE from subscribed RFSP Index value to RFSP index vaule1.
Step 9.
The MME provides the RFSP index vaule1 as the RFSP in use to the eNB in UE CONTEXT MODIFICATION procedure as in clause 8.3.4 of TS 36.413.
Step 10.
Based on RFSP index value 1, the ENB keeps the UE in 4G, and will not direct the UE to 5G bases on subscribed RFSP Index value.
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6.9.2.2  Procedure for triggering UE to move from 4G to 5G  p. 27

Copy of original 3GPP image for 3GPP TS 23.700-89, Fig. 6.9.2.2-1: Procedure for triggering UE to move from 4G to 5G
Figure 6.9.2.2-1: Procedure for triggering UE to move from 4G to 5G
(⇒ copy of original 3GPP image)
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Step 0.
After procedure depicted in clause 6.9.2.1, the UE moves from 5G and registers to EPC via E-UTRAN. The PCF doesn't release AM Policy association for the UE.
Step 1.
The PCF decides to adjust the RFSP index to direct the UE from 4G to 5G. The PCF decision could be based on various inputs. For example:
  • Based on various analytics result from NWDAF (e.g. Observed Service Experience analytics, UE communication analytics, Network Performance analytics for 5G, etc.).
  • Bases on application in use, e.g. the application in use is suitable to be performed in 5G.
The PCF updates RFSP index for the UE to RFSP index value 2.
Step 2.
Then the PCF notifies the UDM/HSS the RFSP index vaule2. If the UDM and HSS are deployed separately then the UDM further notifies the RFSP index vaule1 to the HSS. In a combined UDM+HSS deployment this step (step 2b in Figure 6.9.2.2-1) is not needed or handled in implementation specific way.
Step 3.
The HSS notifies RFSP index vaule2 to the MME serving the UE, e.g. by reusing the Insert Subscribe Data procedure as in clause 5.3.9.2 of TS 23.401.
Step 4.
The MME updates RFSP index in use for the UE from RFSP index vaule1 to RFSP index vaule2.
Step 5.
The MME provides the RFSP index vaule2 as the RFSP in use to the eNB in UE CONTEXT MODIFICATION procedure as in clause 8.3.4 of TS 36.413.
Step 6.
Based on RFSP index value 2, the eNB initiates RRC release procedure, which will trigger the UE access to 5G, then UE performs mobility registration procedure to 5GC.
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6.9.3  Impacts on services, entities and interfaces  p. 28

The solution has the following impacts:
PCF:
  • Notifies the updated RFSP index to the UDM/HSS.
  • Remains the AM policy association for the UE in the 5G&4G interworking scenario when the UE moves from 5G to 4G.
UDM/HSS:
  • Receives the updated RFSP index from the PCF and notifies it to the MME serving the UE during Update Location or as a Standalone Insert Subscriber Data operation.
MME:
  • Receives the updated RFSP index from the HSS and set the RFSP index in use for the UE based on it.
AMF:
  • Remains the AM policy association for the UE in the 5G&4G interworking scenario when the UE moves from 5G to 4G.
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